Fiber-Reinforced Polymer (FRP)

Fiber-Reinforced Polymer (FRP)

Overhead sign structures perform a valuable service to the traveling public. They support the signs that inform drivers well before any action is required, thereby making travel safer. If these structures are not properly maintained, however, they can collapse onto the roadway below and create serious driving hazards.

How can the collapse of sign structures be prevented?

The best insurance for preventing the collapse of overhead sign structures is a comprehensive inspection program. When the New York State Department of Transportation (DOT) launched a sign inspection program in 2000, it found that 10 percent of the State's overhead sign structures were damaged. The most common problem was fatigue cracking in the welds connecting the diagonals and the main chords of the sign structure, with some joints totally severed.

What causes cracks in sign structures?

The causes of cracks are difficult to determine and may stem from several factors. Lack of inspection during fabrication can yield poor-quality joint welds. Insufficient construction supervision may result in internal stresses in an overhead sign structure before the sign is attached. The greatest contributor may be that fatigue design was not a code requirement when many trusses were designed in the 1960s.

Fiber-reinforced polymer (FRP) composite materials have the potential to revolutionize the repair of sign structures with cracked secondary support members. The Federal Highway Administration (FHWA) has researched the use of FRP for more than 20 years, and FRP has been used on a variety of bridges and other highway structures.

Using FRPs to repair cracked overhead sign structures represents one of the latest applications of these strong and durable materials in maintaining the Nation's aging highway infrastructure. FRPs can provide structural integrity to overhead sign supports and prevent them from failing.

How is FRP used to repair overhead sign structures?

The FRP repair method is relatively quick and economical. It is accomplished by cleaning the damaged area of the sign support thoroughly and wrapping FRP around it. Repairs can be done in place, with only the lanes below the repair area blocked off. A typical repair takes three workers 3 hours to complete, at an estimated cost of $3,000 per joint.

Putting It in Perspective
A 2002 survey of sign inspection practices at State transportation agencies found the following:

In several instances, structures have failed and fallen into traffic below.

Cantilever sign supports have the most problems, according to 54 percent of respondents.

Problems cited most frequently were weld defects in sign and pole construction and general fatigue cracking.

Benefits

Costs less than full structural support replacement.

Allows repairs to be done quickly.

Causes less traffic disruption because only lanes beneath repair need to be blocked off.

Successful Applications: Research shows FRP repairs are as strong and as durable as welded joints in tension.

This is recommended, however, only for secondary members. The State should continue to inspect and monitor the repairs to ensure safe performance of the sign structures.

The New York State and Utah DOTs collaborated on a research program to study the feasibility of using FRP to repair overhead sign structures. Samples of cracked joints were salvaged from overhead sign structures that had been taken out of service. The samples were wrapped with FRP and sent to the University of Utah for tensile strength and fatigue tests. Results showed that the repaired joints were as strong and durable as if they had been fully welded. As a result of the research, the New York State DOT has developed a specification for using FRP in overhead sign repair. The specification covers restoration of the tensile capacity of secondary sign structural members, such as internal truss diagonals, and not main members, such as longitudinal truss chords. The specification has been approved for a 5-year lifespan with annual inspections of the repair.

National Deployment Statement

Wrapping FRP around overhead sign supports can prevent failure and lengthen the service life, improve the structural integrity, and repair cracks in the supports.

National Deployment Goal

By 2010, 80 percent of States that have cracking problem in their sign trusses will have used FRP to repair and lengthen the service life of the trusses.

National Deployment Status

A revised plan and goals are required since the manufacturers have indicated that further demand for their product has been very slow.

After the AASHTO TIG closed out this technology and turned it over to the FHWA, the Resource Center volunteered to take the lead with further implementation efforts. The RC Structures Team worked with the HQ HIBT office to develop and distribute a memo to the field in December 2006, soliciting interest in hosting demonstrations of the technology. By February 2007, approximately 30 states had responded, with 7 expressing positive interest in hosting a demo.

Three demos were held in 2007 and an additional one was held in 2008 as a product demonstration showcase. Two states hosted indoor demos on sample sign truss sections, one state hosted an outdoor demo on an in-service overhead sign truss, and one state hosted an indoor and an outdoor demo. Approximately 80 people attended these demos, including representatives from the local FHWA Division Offices; State DOT Central and District Office Bridge Design, Maintenance, and Inspection Engineers, and Traffic Engineers; and from local academia.

At this time, two states and three Canadian provinces have expressed interest in hosting a demo of the FRP technology, and we will be planning to schedule these over the next 2 years.